1. Field of the Invention
The present invention relates to a scanning optical apparatus used for an image forming apparatus such as a laser printer or a laser facsimile apparatus.
2. Related Background Art
As shown in FIG. 1, a scanning optical apparatus used for an image forming apparatus such as a laser printer or a laser facsimile apparatus generally comprises a light source unit E which incorporates a semiconductor laser, a collimator lens, and the like, a rotary polygonal mirror R for deflectively scanning a laser beam L.sub.0, i.e., a collimated beam emitted from the light source unit E, and an imaging lens system F for forming the deflectively scanned laser beam into an image on a photosensitive material N serving as a recording medium on the surface of a rotary drum. The imaging lens system F has a spherical lens F.sub.1 and a toric lens F.sub.2. The rotary polygonal mirror R, the spherical lens F.sub.1, and the toric lens F.sub.2 are accommodated in an optical box H. The light source unit E is mounted on the side wall of the optical box H.
The upper opening of the optical box H is closed by a lid (not shown) after all necessary parts are mounted in the optical box H. A window M for extracting the laser beam defected and scanned by the rotary polygonal mirror R toward the external rotary drum is formed in the bottom wall of the optical box H.
The laser beam L.sub.0 emitted from the semiconductor laser of the light source unit E is collimated by the collimator lens in the light source unit E, linearly focused on the reflecting surface of the rotary polygonal mirror R through a cylindrical lens C, reflected downward by a deflecting mirror G through the imaging lens system F, and extracted from the window M of the optical box H toward the rotary drum. With this arrangement, the laser beam formed into an image on the photosensitive material N on the rotary drum is formed into an electrostatic latent image with the main scanning operation by the rotary polygonal mirror R and the sub-scanning operation by rotation of the rotary drum.
The imaging lens system F has a so-called f-.theta. function of setting the constant scanning speed of a point image formed on the photosensitive material N in the above-described manner. The spherical lens F.sub.1 and the toric lens F.sub.2 are strictly positioned with respect to the optical path of the laser beam (scanning light beam) scanned by the rotary polygonal mirror R, and then fixed in the optical box H by a known technique such as bonding.
In recent years, inexpensive plastic lenses have been developed as the toric lens F.sub.2. A plastic toric lens has an advantage in simplifying the assembly process of the scanning optical apparatus because a positioning projection D is integrally formed at a predetermined position of the toric lens and engaged with positioning pins P.sub.1 and P.sub.2 standing on the bottom wall of the optical box H, resulting in facilitation of positioning the scanning direction.
According to the above prior art, however, the toric lens has an aspherical shape asymmetrical about its optical axis, i.e., the center of the longitudinal direction, though this asymmetry cannot be easily recognized from the outer appearance. Therefore, the toric lens is often erroneously mounted upside down in the optical box.
Depending on the type of the scanning optical apparatus, the scanning direction of the scanning light beam is reversed to that described above, so that it is sometimes required to mount a toric lens having the f-.theta. function upside down. When bearing surfaces (reference surfaces) for mounting the lens to the bottom wall of the optical box are formed on both the upper and lower end faces of the toric lens, the toric lens can also be applied to a scanning optical apparatus of another type. For this reason, the application range (application purpose) of the part can be made wide to largely reduce the unit costs of parts. However, when similar bearing surfaces are formed on both the upper and lower end faces of the toric lens, upside-down mounting further increases.